A wavelength dispersing device is an optical device for spatially separating spectral components of light for subsequent measurements and, or for further routing or switching of these spectral components.
A wavelength dispersing device is a key component of an apparatus for measuring a spectrum of light, or an optical spectrometer. Optical spectrometers are used for remote sensing of temperature, determining chemical composition and concentration of chemical compounds, identifying substances, determining parameters of optical channels in an optical communications network, and other applications. A wavelength dispersing device is also one of the key components of a wavelength selective optical switch for independent wavelength-selective switching of individual wavelength channels in an optical communications network. An optical channel equalizer for dynamic equalization of optical power levels of the individual wavelength channels and an optical gain equalizer for dynamic equalization of optical gain levels of the individual wavelength channels in an optical amplifier can also be based on a wavelength dispersing device.
Despite proliferation of wavelength dispersing technologies based on compact planar lightwave circuits or fiber Bragg gratings, a technology based on free space optics such as a diffraction grating or a dispersive prism still remains one of the most frequently used and relied upon for high levels of performance and high reliability. A diffraction grating diffracts a light beam impinging thereon into a fan of narrowband sub-beams at individual wavelengths. A diffraction grating, although invented about two hundred years ago, has an advantage of a high achievable diffraction efficiency and a low achievable wavefront distortion. A low polarization sensitivity of a diffraction grating is also attainable in some cases.
With regards to application of diffraction grating based wavelength dispersive devices for optical communications networks, a folded symmetrical 4-f free-space optical configuration taught in U.S. Pat. No. 6,498,872 by Bouevitch et al., with an optional field-flattening optical wedge taught in U.S. Pat. No. 6,760,501 by Iyer et al., both assigned to JDS Uniphase Corporation and incorporated herein by reference, allow construction of dynamic gain equalizers for equalizing optical power values of individual wavelength channels, wavelength blockers for completely blocking any subset of a full set of the wavelength channels, and wavelength selective optical switches for performing the abovementioned wavelength channel switching function.
As an example, referring to FIG. 1, a prior-art optical configuration of a wavelength selective optical switch (WSS) 10 is shown. The optical elements of the WSS 10 are: a front end 11 for launching and receiving free-space optical beams having a plurality of wavelength channels, a concave mirror 12 for focusing and collimating optical beams, a diffraction grating 13 for spatially dispersing an input optical beam into the wavelength channels and for combining the wavelength channels into an output optical beam, a field-flattening wedge 14 for reducing spherical aberration of the WSS 10, and an optical switching engine 15 for selectively switching individual wavelength channels from an input optical port 16 to an output optical port 17, wherein both the input and the output ports 16 and 17 are optically coupled to the front end 11. The optical switching engine 15 has an array of beam directing elements, or “directors”, which can be either micro-electro-mechanical system (MEMS) micromirrors or liquid crystal (LC) pixels.
In operation, an input optical signal is launched into the input optical port 116 of the front end 11 optically coupled by the concave mirror 12 to the diffraction grating 13, which disperses an incoming optical beam 18 into narrowband sub-beams 19 carrying individual wavelength channels. Throughout the specification, the term “narrowband” is understood as having a narrow wavelength range as compared to a wavelength range of the light beam. By a way of non-limiting example, a wavelength range, or a bandwidth of a single wavelength channel could be 0.4 nm, whereas the wavelength range of the light beam 18 could be 32 nm. The concave mirror 12 couples the narrowband sub-beams 19 to the optical switching engine 15, which spatially redirects the narrowband sub-beams 19. Upon reflecting from the optical switching engine 15, the narrowband sub-beams 19 are collimated by the mirror 12, recombined by the dispersive element 13, and focused by the mirror 12 back into the front end 11 coupled to the output optical port 17. Depending upon the state of individual pixels, not shown, of the optical switching engine 15, the individual wavelength channels may be attenuated, switched to the output port 17, or suppressed. The footprint of the WSS 10 of FIG. 1 for a 100 GHz channel spacing is approximately 2×3 inches. A detailed description of operation of the WSS 10 shown in FIG. 1 can be found in the abovementioned US Patent documents.
Although WSS 10 has a folded optical path as explained, which allows an optics footprint reduction, a market pressure exists to further reduce the size of the optics of WSS devices and the wavelength dispersing devices they are based upon. This market pressure is caused in part by competing planar technologies and results from a desire of optical communication system providers to offer higher levels of functionality at the same or smaller size and cost of their circuit packs.
One known way to reduce the overall size of the WSS 10 is to reduce the focal length of the concave mirror 12. However, the spacing of optical wavelength channel sub-beams along the switching engine 15 has also to be scaled down in proportion to the focal length of the concave mirror 12. Switching engine technologies have limits of the minimum practical size of the individual directors; therefore, at a given angular dispersion of the dispersive element 13, a limit exists for the minimum focal length of the concave mirror 12. Furthermore, to maintain a given spectral resolution expressed as a ratio of the wavelength channel spacing to a spot width of the sub-beams 19 at the switching engine 15, the spot width must also scale with the focal length of the concave mirror 12. This means that the numerical aperture (NA) of the sub-beams 19 in the dispersion direction must scale inversely with the focal length of the concave mirror 12. As the beam NA becomes larger, optical aberrations become more problematic.
Another known footprint reduction technique of a free space optical wavelength dispersing device is to introduce folding mirrors into an optical layout of the wavelength dispersing device. Turning to FIG. 2, a prior-art monochromator 20 of U.S. Pat. No. 6,597,452 by Jiang et al. is presented. U.S. Pat. No. 6,597,452 is incorporated herein by reference. The monochromator 20 is used for selecting one monochromatic component of a polychromatic light, for example, in a spectrometer application. The monochromator 20 has a front end 21, a concave mirror 22, a diffraction grating 23, and a folding mirror 24. In operation, a diverging optical beam 25 emitted by the front end 21 impinges onto the concave mirror 22 that collimates the diverging optical beam 25 into a collimated beam 26 and directs the collimated beam 26 towards the folding mirror 24. The folding mirror 24 directs the collimated beam 26 towards the diffraction grating 23, which reflects one monochromatic component 27 to propagate back towards the front end 21 for outputting from the monochromator 20. The monochromator 20 is tuned by rotating the diffraction grating 23 as indicated by arrows 28.
Yet another known way to reduce a footprint of a free space optical wavelength dispersing device is to double-pass light through a transmission diffraction grating (T-DG), to effectively double the angular dispersion of light, so that the focal length of a focusing element of the wavelength dispersing device can be reduced. Referring now to FIG. 3, a double pass arrangement 30 for a T-DG 31 is shown. This arrangement is taught in U.S. Pat. No. 6,765,724 by Kramer, which is incorporated herein by reference. An incoming beam 32 is diffracted by the T-DG 31 to form narrowband sub-beams 33A and 33B at an angle ΔΘ therebetween. The narrowband sub-beams 33A and 33B are reflected by a mirror 34 to propagate back towards the diffraction grating 31, which further diffracts the sub-beams 33A and 33B to form narrowband sub-beams 35A and 35B, respectively, at an angle 2ΔΘ therebetween. Thus, effective wavelength dispersion of the T-DG 32 doubles upon double passing the light beam 32 through the T-DG 32.
One drawback of the approach represented by FIG. 3 is that the T-DG 31 creates multiple reflections as a result of the diffraction occurring both in reflection and transmission directions. As a result of these multiple reflections, multiple stray light beams are created, resulting in deleterious optical cross-talk.
It is an object of the present invention to provide a wavelength dispersing device for use in optical spectrometers and wavelength selective optical switches, which is free from the above mentioned drawbacks. Advantageously, a wavelength dispersing device of the present invention achieves a high degree of space utilization and high spectral dispersion, without associated excessive optical aberrations or stray light-induced optical cross-talk. This enhanced optical performance at a compact size is attained without having to rely on a large number of additional optical elements.